Difference between preterite and past participle

That a considerable economy has been effected in verb conjugation may
be seen chiefly in the invariable negative form ain’t, which serves
to negate the present tense of both to be and to have:

He ain’t there = He isn’t there

He ain’t been there = He hasn’t been there

The preterite of to be takes the invariable form was (I was etc,
but also we was etc), though the present tense remains, at present,
identical with that of the verb in Bourgeois English (BE). In strong
verbs, preterite and past participle are usually the same in form – as
in I done it; I ain’t done it – though the choice of form from the two
available in BE follows a seemingly arbitrary procedure:

I seen it; I’ve seen it (BE past part.)

I done it; I’ve done it (BE past part.)

I ate it; I’ve ate it (BE pret.)

I swum; I’ve swum (BE past part.)

I forgot; he’s forgot (BE pret.)

I wrote; he’s wrote (BE pret.)

I fell; it’s fell (BE pret.)

I drunk it; I’ve drunk it (BE past part.)

Considerations of syllabic economy seem to determine the preference
for the shorter of the two forms available (wrote, not written;
forgot, not forgotten), but there is no ready explanation of seen
for saw and done for did in the demotic tradition embodied in
Workers' English. It should be noted also that there has never been,
in that tradition, any impulse to level strong verbs under weak forms
(I eat; I eated etc); the ablaut transformation – as also in certain
nouns – is rooted deeply in the language of the workers, and the
rational weakening of strong verbs, desirable to the regularizing
philologist, would find no acceptance among WE speakers, who would
consider such formations as ‘childish’.

– 1985 by Anthony Burgess

In these part of the book the author explains the rules in Workers' English and Burgeois English (These are two languages used in an imaginary conutry). In those languages the "government" tries to eliminate as many words as it can for "simplifying and rationalising" english in the country. There are no prior mentions of those languages in the book. The explanations start from here.

The first part I separated with a line is simple. I understand what the author says.

But in the second part I don't understan what does author mean by "choice of form from the two available" (emphasis in text is mine). What are two available choices? Past simple and past participle forms?

On the examples I see that when he uses past participle it is noted as BE past part. (as in I seen it; I’ve seen it ) and when he uses past simple it is noted as BE pret. (as in I ate it; I’ve ate it). What I don't understand here is: Are both I ate it and I’ve ate it in the preterite form (because he has written BE pret. in front of them)? Or what does he mean by BE pret. and BE past part..